UK Transformer Source + Champ questions

[Tube Guy]

Can anyone recommend a UK source for a Champ power supply (370V, 6.3V, 5V), 240V primary?

I'd also read that the old Fenders used undersized power filter caps that were only just able to suppress mains hum (the Champ 5C1 has 3 x8uF). Should I use larger ones, if so how big and which ones do I change?

And finally, I plan to add a switchable tone stack. Should I use a high insertion loss blackface design or a Baxandall? My thinking is that the blackface will lower the gain and give me a cleaner sounding Champ whereas the Baxandall will do a better job of cutting or boosting the mids (by adjusting treble and bass up or down together).

Any advice appreciated.

 

[John Phillips]

Not sure. I used a step-down transformer mounted in the bottom of the cab on my 5F1 Champ, which is easier and maintains the characteristics of the original PT.

I also replaced the filter caps. It's a long time ago but I think I used 10uF (the 5F1 also has a 16uF as the first filter, I used a 22)... certainly there is no disadvantage to increasing the value slightly. There's probably no disadvantage to increasing it a lot, as long as you don't pull too much current from the rectifier on start-up.

You should change all the electrolytic caps really. This certainly means all the filter caps, and I would replace the cathode bypass cap for the 6V6 too. If it shorts it will take out the 6V6 and stress the OT.

Not sure about the tone stack... I'd keep it as simple as possible.

 

[Tube Guy]

Thanks John

I should also have mentioned that it's a Champ project. I'm building a 5C1 since I heard several tweed Champs recently and liked that one the most.

I'll do what you suggest and increase the filter caps, probably to 22,10,10 or therabouts. Presumably 40 would be about as high as you'd need to go?

I have a faceplate with Volume, Treble and Bass controls so I thought I'd include a switchable tone stack. I just need to decide which type.

 

[John Phillips]

To be honest whenever I've done projects like this I've used scrap parts - cheap (or free) and have a 'vintage vibe' built in! Old tube radios and the like are excellent sources for both PTs and OT for this kind of low-watt amp. The original Champ parts were pretty much the same anyway.

If there's a vintage-radio type shop or repairer in your town that's probably the best bet - most of these guys have tons of the stuff.

Filter cap values are not at all critical (not least because the caps are usually 20% tolerance, so an "8" and a "10" actually could be exactly the same value anyway), but I probably wouldn't go much more than double the original. There's no real point, and you do risk overloading the PT and rectifier.

For the tone stack I probably would go for a BF type. I'm not sure Baxandall types really suit guitar amps... I'm not really sure why, but they do seem to be rarely used. You could always try it and find out.

 

[Tube Guy]

Thanks again John.

I might try some of the U.S. transformer manufacturers first since I'm not sure how easy it would be to find the right spec radio transformer.

The BF tone stack seems to be the way to go then.

Thanks
Brian